1. Field of the Invention
The present invention relates to a waveform generating circuit for making certain modifications to a triangular wave signal, and to a spread spectrum clock generator using the waveform generating circuit.
The spread spectrum clock generator refers to a clock generator for reducing electromagnetic radiation by varying the clock frequency by the so-called spread spectrum clock technique.
2. Description of the Related Art
In the field of the digital circuit, it is known to vary the clock frequency slightly to reduce the electromagnetic radiation caused by the clock.
To obtain the frequency modulated clock, it is generally performed to input a triangular wave to a VCO (voltage-controlled oscillator).
FIG. 1 shows an example of a triangular wave generating circuit conventionally known. The input signal to the circuit is a rectangular wave, and complementary switch 22 and switch 23 which are driven through an inverter 20 repeat turning on and off alternately. Thus, a capacitor (CAP) 25 is supplied with the current from a current source 21 and the current from a source 24 alternately. In this way, the CAP 25 repeats charge and discharge, thereby outputting a triangular wave.
Japanese patent application laid-open No. 06-242851 discloses the following object and configuration (in abstract and FIG. 1). Its object is “to make the interference visually inconspicuous on a screen caused by spurious radiation being fed to a TV tuner, which is generated by digital circuits such as a microcomputer and the like”. Its configuration is “a circuit for generating a clock pulse of a microcomputer, which includes pulse generating means for generating a pulse signal with a fixed frequency; and random modulation means for modulating the pulse signal at random the pulse generating means generates, wherein the random modulation means includes a triangular wave generator; a frequency modulator for carrying out frequency modulation of the triangular wave produced by the triangular wave generator; and a frequency converter for carrying out frequency conversion based on the output of the frequency modulator and a signal obtained from the pulse generator, and wherein the output of the frequency converting means is used as the clock pulse”.
However, when the frequency characteristic applied for the input to the modulation circuit or the oscillator is insufficient for achieving the frequency modulation by the triangular wave as described above, there arises a problem in that the vertices of the triangular wave round, and ideal spread spectrum is not achieved.
When the spread spectrum is brought about by the triangular wave with such a shape, a spectrum is observed which has higher level portions at both ends of the spread spectrum than in other regions, which presents a problem in that the effect of the spread spectrum is impaired.
On the other hand, even if the frequency modulation is carried out using a triangular wave with an ideal shape, since the EMI (electromagnetic interference) measurement is made based on the finite frequency resolution and measurement integral time, the frequency transition stays at frequencies near the vertices of the triangular wave longer than in other frequency ranges to be measured. As a result, a problem arises in that the spectrum is observed which has the higher level portions than in the other regions (see FIG. 7A).
To circumvent such a problem, a technique is conceived which carries out modulation after changing the waveform for the frequency modulation to a particular shape. The technique, however, must pay extra cost in both a circuit area and cost (see Electromagnetic Compatibility EMC, May, pp. 22-28).
In view of the foregoing problems, a first object of the present invention is to provide a waveform generating circuit for generating a modified triangular wave signal suitable for being input to a frequency modulation circuit such as a VCO (voltage-controlled oscillator).
A second object of the present invention is to provide a spread spectrum clock generator capable of improving the spread spectrum effect with little increase in the circuit cost by modifying the shape of the triangular wave used for the frequency modulation with a simple method.